1. Field of the Invention
The present invention relates to an ignition coil for an internal combustion engine, and in particular, although not exclusively, relates to an ignition coil for an internal combustion engine having an iron core which is inserted through a primary former supporting a primary, coil.
2. Description of the Related Art
As disclosed in Japanese Patent Application Laid-Open No. 56-42316 (1981), a now often used ignition coil for an internal combustion engine is constructed by entirely enclosing a primary coil, a secondary coil, their associated bobbin former and an iron core within a synthetic resin.
In recent times it has become desirable to make the ignition coil of small size and light weight so that it is required to discard the enclosure entirely surrounding the iron core, coils and synthetic resin and instead to employ a structure in which the iron core is exposed. In particular, such a requirement is extremely desirable in an ignition device for a DIS (Direct Ignition System) which uses one ignition coil for one or two ignition plugs.
Such an ignition coil is disclosed in Japanese Patent Application Laid-Open No. 55-103712 (1980).
It is appreciated that the engine compartment where the ignition coil is mounted, is exposed to open air and directly receives the influence of the external atmosphere. Therefore, when a car runs on a road near the sea or one scattered with salt to melt snow in the winter, the engine compartment is filled with external air containing components of salt and water.
When such external air containing salt and water enters into the clearance between the primary former and the iron core inserted therethrough, the problem arises that the iron core rusts to produce a corrosion expansion so that stress due to the corrosion expansion is transmitted to the coil through the former and a crack occurs in a filler agent between the respective bobbins of the primary and secondary coils. The ignition coil then breaks down.
It is an object of this invention to provide an improved ignition coil. It is a further object of this invention to provide an ignition coil in which stress caused by corrosive expansion of the iron core is reduced.